Why dark vignette looks posterized?
What you are experiencing is a quantization error called banding.
Most monitor can only display 8 bits per color or 256 tones from dark to bright. There are just not enough levels to do a subtle gradient between two dark tones or similar colors.
It is a limitation that will show in most monitors (unless you have one that can do 10 bit display).
As a workaround, you can play with the dither settings in the Post Processing settings for the scene.
(from the blender Manual)
Dithering

Dithering is a technique for blurring pixels to prevent banding that is seen in areas of gradients, where stair-stepping appears between colors. Banding artifacts are more noticeable when gradients are longer, or less steep. Dithering was developed for graphics with low bit depths, meaning they had a limited range of possible colors.